1. Field of the Invention
The invention relates to a fully hydraulic steering system compensating for steering angle error, in which, on displacement of a steering handwheel, a steering handwheel angle and a steering motor angle and also the steering handwheel speed are ascertained, and the fluid flow between a steering control unit and a steering motor is changed in dependence on the steering angle error; the invention also relates to a fully-hydraulic steering system, in particular for vehicles, having a steering motor, a steering control device connected thereto by way of a fluid path and operable with the assistance of a steering handwheel, a steering handwheel sensor, a steering motor sensor, a processing device, and an auxiliary fluid path that branches off from the fluid path and by-passes the steering control device and has a valve arrangement for controlling the flow of auxiliary fluid flowing through the auxiliary fluid path, which valve arrangement is operable by the processing device, the processing device comprising a difference-generating means for ascertaining the steering angle error and a differentiator for ascertaining the steering handwheel speed.
Steering systems are generally used in vehicles. Here, steered wheels or steering means are swung through an angle which corresponds to a specific angle of rotation of the steering handwheel. In the present application, however, the term "steering system" is intended to be understood in a wider sense. It also relates to the control of operating means in which a displacement of a work motor is effected proportionally with respect to the actuation of an operating lever. The operating lever is referred to hereinafter for the sake of simplicity as the "steering handwheel". The movement of the steering handwheel need not be restricted to rotary movements. When an operating lever is used as the steering handwheel, it is also possible to measure translational movements. The steering handwheel angle then corresponds to a linear distance. For example, a steering motor can bring an extension arm of a driven machine into a certain position which is dependent on the position of the steering handwheel. To facilitate understanding, however, the following description refers to a vehicle steering system.
2. Description of the Prior Art
In the case of a steering system known from U.S. Pat. No. 4,703,819, the rear wheel of a three-wheeled fork-lift truck is fully-hydraulically controlled. In that case, the position of the rear wheel is intended to correspond to the angular position of the steering handwheel. This state is identified by a button on the steering handwheel. Deviations of the two positions from one another may occur, for example, as a result of leakages. The deviation between the angular position of the steering handwheel and the angular position of the steered wheel is referred to hereinafter as the steering angle error. To compensate for the steering angle error, in the known steering system provision is made for a part of the fluid flow to the steering motor from the steering control unit to branch off directly to the tank, so that the steering handwheel can be turned without the steering motor being displaced further. It is thus possible to achieve correspondence between the steering handwheel position and the steering motor position. One construction of the known steering system contains a manually-operable valve, which can be operated in the neutral position of the steering motor. After operation of the valve, the steering handwheel can also be brought into the neutral position, without the rear wheel being steered with it. In another construction, the steering angle error compensation is effected under the control of a processing device. In that case, the steering handwheel position and the steering motor position are measured with the assistance of sensors. The measured values are processed by a microcomputer which operates an electromagnetic valve that connects to the tank a portion of the line from the steering control unit to the steering motor. Compensation of the steering angle error is effected, however, only if the speed of the steering handwheel exceeds a predetermined minimum speed or if the steering angle error has a specific minimum value. Below these values, there is a risk that, although an operator is able to turn the steering handwheel, this would not lead to a change in direction of the vehicle.
U.S. Pat. No. 4,792,008 describes a similar system in which additionally the directional dependency of the rotation of the steering handwheel is taken into account.
JP 60-261 778 A also describes a fully-hydraulic steering system with steering angle error compensation. In that case, to compensate for a steering angle error, additional hydraulic fluid is supplied to the steering motor with the aid of an additional pump, so that the positions of steering handwheel and steering motor can again be made to correspond.